Collective excitations in nanoscale thin alkali films: Na/Cu(111).

The relationship between electron quantum confinement and the energy dispersion of the surface plasmon in nanoscale thin Na layers adsorbed on Cu(111) at room temperature have been studied by high resolution electron energy loss spectroscopy. Screening effects due to electron quantum confinement occurring in this system cause the lowering of the surface plasmon frequency and, moreover, make the energy range of its dispersion curve larger than in thick alkali films. Landau damping of the plasma excitation was unexpectedly very efficient at small momenta. The dispersion curve of the Na surface plasmon was found to depend on the primary electron beam energy. Multipole surface plasmon at 4.70 eV was observed only for higher impinging energies.

Electronic structure of cluster assembled nanostructured TiO(2) by resonant photoemission at the Ti L(2,3) edge.

The electronic structure of cluster assembled nanostructured TiO(2) thin films has been investigated by resonant photoemission experiments with photon energies across the Ti L(2,3) edge. The samples were produced by supersonic cluster beam deposition with a pulsed microplasma cluster source. The valence band shows resonance enhancements in the binding energy region between 4 and 8 eV, populated by O 2p and hybridized Ti 3d states, and in the region about 1 eV below the Fermi level associated with defects related Ti 3d states. The data show that in as-deposited films Ti atoms are mainly fully (sixfolds) coordinated to oxygen atoms in octahedral symmetry and only a small fraction is in a broken symmetry environment. Since resonant photoemission is closely linked to the local electronic and structural configurations around the Ti atom, it is possible to correlate the resonant photoemission intensity and lineshape with the presence of defects of the films and with the degree of hybridization between the titanium and oxygen atoms.

Evidences of alkali-induced softening of the oxygen-substrate bond.

The interaction of oxygen with alkalis (Na, K) on Ni(111) was studied by high-resolution electron energy loss spectroscopy. Loss measurements revealed for the first time a softening of the O-Ni bond and, simultaneously, a strengthening of the alkali-Ni bond in the alkali+O coadsorbed phase, in perfect agreement with recent theoretical calculations. The weakening of the O-Ni bond was ascribed to the alkali-induced filling of the O 2p(z) antibonding orbitals. Different physical mechanisms were discussed for explaining the strengthening of the alkali-substrate bond whenever alkalis are coadsorbed with O adatoms.

High resolution electron energy loss measurements of Na/Cu(111) and H2O/Na/Cu(111): dependence of water reactivity as a function of Na coverage.

Collective electronic excitations occurring in Na layers grown on Cu(111) and in H2O/Na/Cu(111) have been investigated at room temperature by high resolution electron energy loss spectroscopy. Loss spectra taken for a coverage between 0.55 and 0.70 ML of Na are characterized by a feature at 3.0 eV assigned to a Mie resonance. Further increasing the Na coverage leads to the appearance of the Na surface plasmon at 3.9 eV. Water molecules dissociate on Na layers as shown by the appearance of the OH-Na vibration. Upon water adsorption, relevant effects on both electronic excitations and vibrational modes were observed as a function of Na coverage.

Nanocrystalline metal/carbon composites produced by supersonic cluster beam deposition.

In this work we show that supersonic cluster beam deposition is a viable method for the synthesis of nanocrystalline metal/carbon composites. By assembling carbon and metallic clusters seeded in a supersonic beam, we have grown films consisting of metal nanoparticles embedded in a nano-structured carbon matrix. Samples containing 3d transition metals (Ti, Ni) and noble metals (Au, Pd, Pt) with different metal abundances, particle size and dilution have been characterized by transmission electron microscopy. The influence of different metals on the structure of the carbon matrix has been investigated. Spatially resolved ultraviolet photoemission electron spectroscopy showed substantial surface oxidation of 3d transition metal clusters. On a micrometric scale, the spatial distribution of the metallic nanoparticles appeared to be homogeneous.

Kramers-Krönig analysis of modulated reflectance data investigation of errors.

The errors introduced in Deltaepsilon(2) spectra by Kramers-Krönig analysis of modulated reflectivity data are investigated using an analytical model. It is found that the energy position of singularities is always reproduced with good accuracy even if the experimental spectrum of DeltaR/R is cut barely above the last structure of interest. This procedure is instead completely insufficient when a quantitative line shape analysis is required. In such cases data up to very high energy are required for a meaningful analysis. Errors due to other sources, like baseline shifts or inaccurate static optical constants, are also investigated.